In the search for suitable materials from which to construct high energy density solid state batteries, one of the principal obstacles has been the provision of a suitable electrolyte. A variety of approaches have been tried heretofore. The one which received the most attention among those prior approaches is the one based on polymer solvents in which an optimized amount of ionic salt is dissolved in the polymer solvent (See Armand et al., U.S. Pat. No. 4,303,748; Andre et al., U.S. Pat. No. 4,357,401; and Kronfli et al., U.S. Pat. No. 5,009,970). Other approaches, which possessed both specific advantages and disadvantages, involved glassy solid electrolytes, and certain plastic crystal or disordered crystal electrolytes. None of these approaches, nor, indeed, any of the prior art approaches, obtain all the properties generally recognized as prerequisites to the successful development of a high power solid state battery, namely, (1) high ionic conductivity (about 10.sup.-3 .OMEGA..sup.-1 cm.sup.-1 or above); (2) conductivity by lithium cations (to avoid undesirable cell polarization problems); (3) a rubbery consistency (to permit the deformation of the electrolyte as needed to accommodate volume changes during charging and discharging cycles); (4) a wide electrochemical window (to permit the utilization of anode/cathode combinations which provide high voltages); and (5) good adherence to the electrode surfaces (to prevent mechanical/electrical problems that could otherwise develop during charging and discharging cycles).
Each substance heretofore developed for solid electrolyte purposes possesses only a limited number of the above-identified desiderata. None achieved them all. For instance, the so-called superionic glass electrolyte, exemplified in the most successful case by Li.sub.2 S--LiI--Y (where Y is a Lewis acid such as P.sub.2 S.sub.5, B.sub.2 S.sub.3, SiS.sub.2), achieves some of the above listed properties namely, 1,2,4 and 5 but is quite brittle and totally lacks the desired rubbery consistency. Examples of this type of electrolyte are described by Malugani et al. in U.S. Pat. No. 4,331,750 and by Akridge in U.S. Pat. No. 4,585,714.
The prior art salt-in-polymer approach mentioned above, satisfies three of the desiderata namely, 3,4 and 5, but fails miserably with regard to properties 1 and 2. For instance, neither of two recent U.S. Patents dealing with salt-in-polymer electrolytes reported a room temperature conductivity greater than 1.times.10.sup.-5 .OMEGA..sup.-1 cm.sup.-1 for solvent-free or plasticizer-free systems (See: Kronfli et al., U.S. Pat. No. 5,009,970; Knight et al. U.S. Pat. No. 4,737,422). One prior art effort to rectify the poor conductivity of the salt-in-polymer electrolyte involved the addition of low molecular weight plasticizers to the mixture (See: Koksbang et al. J. Power Sources 32, 175, (1990)) . However, improved conductivity was achieved at the expense of introducing unwanted volatile components into the electrolyte making the electrolyte subject to explosion if overheated, and leading to unwanted reactions between plasticizer and lithium anode. Since the solubility of lithium salts in the polymer electrolytes is predicated upon attraction between the lithium cations and the solvating groups in the polymer, these electrolytes further suffer from the fact that the lithium is the less mobile cation. This means that the cation conductivity desideratum, identified as "2" above, is never achieved except in the poorly conducting, single mobile ion polymers which are described by Noda et al. in U.S. Pat. No. 4,844,995. It is believed that it is fundamentally unlikely that this problem can be rectified in the usual polymer/Li salt type of medium. Claims have been made that the problem can be somewhat reduced by using plasticized polymers although no verification of these claims has been found. Exemplary polymer-in-salt type electrolytes are disclosed in U.S. Pat. Nos. 4,303,748; 4,357,401; 4,585,714; and 5,009,970.
A major improvement, which overcomes the deficiencies of the aforementioned polymer/Li systems, is disclosed by Angell and Liu, U.S. patent application Ser. No. 07/901,669 filed Jun. 22, 1992. Disclosed therein is a novel and unique high-conductivity lithium-containing electrolyte comprising a viscous liquid or rubbery solid comprised mainly of lithium salts which obtain conductivity by the decoupled motion of the lithium ions which remain freely mobile even in a glassy state and at temperatures below -20.degree. C.
While the Angell/Liu electrolyte solves the aforementioned disadvantages to some degree, a continuing desiderata in the art of battery construction is the generation of higher conducting electrolytes with still better thermal, chemical, and electrochemical stabilities.
As is apparent, a great need exists for the development of an improved electrolyte which obtains all of the desiderata listed above without the acquisition of unacceptable deleterious properties. It is toward this goal that the present invention is directed. A preferred embodiment of the present invention obtains all of the desiderata listed above and yields conductivities on the order of 10.sup.-4 .OMEGA..sup.-1 cm.sup.-1. The electrolyte of the present invention obtains an even greater conductivity advantage at temperatures above room temperature. Further, in certain composition realizations, the present invention provides a predominantly Li.sup.+ -conducting (i.e. cation decoupled) viscous liquid electrolyte suitable for use in porous polymer or conventional paste electrolytes, to obtain a conductivity which may reach 10.sup.-3 .OMEGA..sup.-1 cm.sup.-1. These conductivities, which are due to decoupled Li.sup.+ mobility as shown elsewhere (Angell et al., Nature, 362, 137-139, (Mar. 11, 1993)), are comparable with, or better than, those of any previously described ambient temperature molten salt electrolytes containing Li salts, such as those described by Cooper and Angell, Solid State Ionics, 9 & 10, 617, (1983) or by Takami et al., (Chem Abs 118: 195131d, Jpn. Kokai Tokkyo Koho JP 04,349,365 [92,349,365] CI. HOIM10/40.03 Dec. 1992. Appl. 91/120,836. 27 May 1991: 4 pp). Neither of the latter electrolytes fall within the scope of the present application because they owe their high room temperature conductivities to the lowering of T.sub.g obtained by the inclusion of considerable mole fractions of non-lithium salts, namely, organic salts (Tetraalkyl ammonium or otherwise-substituted ammonium salts). As our earlier work cited above shows, inclusion of such organic salts destroys the decoupling of the Li.sup.+ ion motion hence leads to undesirably low Li.sup.+ transport numbers.